Multicolor Photoionization of Atoms with an IR Laser and Its Higher UV Harmonics

Abstract

The recently observed generation, by samples of atomic rare gas, of high-order harmonics of the fundamental frequency of an infrared laser opens the possibility to have at one’s disposal a table-top source of coherent and pulsed VUV, (or even soft X-ray) radiation1–3. The main motivation of the present paper is to address the possibility to perform a new class of (N+1)-color (N≥1) photoionization experiments, which fully exploit the unique characteristics of high-order harmonic radiation. Among these characteristics is the fact that the harmonic emission spectrum is constituted of coherent UV radiation lines with equally spaced frequencies and comparable intensities, the spacing between two lines being twice the frequency ωL of the infrared laser which has been used to generate the harmonics. The idea is to expose simultaneously an atomic target to the fields of a high order harmonic radiation with frequency ωH and the fundamental of the laser ωL. Then, if the harmonic order is high enough, i.e. ωH >EI, where EI is the ionization energy of the atom, Laser-Assisted-Single-Photon-Ionization (LASPI) can be observed. Experimental observations have already been reported4,5.